This invention relates generally to plasma reactors used in etching and other processes and, more particularly to techniques for controlling direct current (dc) bias in such reactors. In a dry etching process typically used in the fabrication of integrated semiconductor devices, process gases are supplied to a reactor chamber and radio-frequency (rf) energy generates and sustains a plasma cloud within the chamber. Ions in the plasma cloud bombard a workpiece, which is usually a semiconductor wafer located in the chamber immediately adjacent to the plasma, or in a separate processing chamber into which ions from the plasma are drawn. The ions either etch the workpiece or assist the etching, and the etching process may be made selective by patterning a protective coating applied to the workpiece prior to etching.
In general, there are three types of plasma generation approaches: capacitive, inductive, and microwave. In the more conventional capacitive plasma approach, the plasma is formed between a pair of parallel plate electrodes, to which radio-frequency (rf) power is applied, to one or both plates. A variant of the parallel plate approach is the magnetically enhanced reactive ion etch (MERIE) plasma generation apparatus, in which a magnetic field enhances the formation of ions in the plasma. Inductive plasma generators use an inductive coil, either a planar coil, a cylindrical coil or any of various other types of coils to deliver rf power into the plasma chamber. A separate rf generator supplies energy to at least one plate electrode in the chamber, to control ion energy and direction.
A well known phenomenon in plasma reactors is the generation of a direct current (dc) bias between the plasma and a lower electrode to which rf power is applied. The dc bias accelerates positive ions in the reactor chamber toward the lower electrode, to which a semiconductor wafer is secured for processing. The energy of the ions accelerated from the plasma is one of the most important factors that determine the rate at which etching of the wafer takes place. Another important factor, of course, is the density of the plasma. The dc bias generated on the lower electrode varies, as might be expected, with the rf power applied to the electrode, and this parameter is sometimes used to control the dc bias, and thereby control the plasma energy and the etch rate. In a conventional reactive ion etch process, the lower electrode is also used to couple energy capacitively into the chamber, to generate and sustain the plasma. In this case, the rf power does not provide an independent control of plasma energy, since it also affects the density of the plasma. Typical dc bias levels in a magnetically enhanced reactive ion etch (MERIE) chamber range from -300 v to -700 v and operating pressures are 50 to 300 mTorr, at powers of 500 to 1000 watts.
A high dc bias has the principal disadvantage that the highly energetic ions it produces tend to cause unwanted damage to the wafer being etched and underlayer sputtering. Related processing problems included back-sputtering of etched material, lower selectivity of etching, and difficulties in forming reliable metal contact layers. Adding a magnetic field reduces the dc bias considerably, but not enough to provide a low damage, high selectivity, and high etch rate process. If the magnetic field is raised too high, above about 40 to 60 gauss (dependent upon the specific process used), device charge-up becomes a significant concern. Device charge-up is caused as a result of ions and electrons in the plasma drifting in opposite directions under the influence of the magnetic field, and a consequent non-uniform charge distribution in the plasma and in the wafer where accumulated charge breakdown occurs.
Inability to control the dc bias places an upper limit on the rf power applied to the reactive ion etch chamber, because this is the only way to limit damage caused by a high dc bias. Limiting the rf power necessarily limits the etch rate, so the chamber can only be used for one type of process. For example, a chamber designed to have a low dc bias can be used for low-damage processes, but a different chamber design is needed for a process needing higher power and a higher etch rate.
Clearly it would be desirable to have an etch reactor that could be operated at a selected lower dc bias, to avoid the problems caused by a high dc bias, but still be able to operate at relatively high etch rates. The present invention achieves this goal and provides additional advantages.